First, your distance coach has no idea whether you can run <150. What he is doing is trying to fit you into his training paradigm and he figures if a kid with 56 400m speed can bust out a 2:01 800m, putting in 60 or so miles/week, imagine how fast this other kid who splits a 46 will be if he trains just like the slowpoke. It's overly simplistic thinking, really. The truth is you are probably just as well off listening to the 400/800 coach as the distance coach, and the better your basic speed, the less likely it is you need high volume training to run a great 800m. Bottom line is either one can help you or ruin you, and if you want to get to the NCAA meet or the world list, you're gonna have to figure out what works best for you.
You do have some room for improvement, and I consider 46 4x400m speed, more or less the minimum for gaining a spot in a world championship 800m final these days. The real question is how to attack for stamina gains in your individual case. My advice would be to explore the middle ground for now. Truth is if you can't hit 118-119 for 600m solo on 20-25 miles/week, you are probably better off racing the 400. Moreover, if you do increase your mileage, the mileage you do still needs to be tailored to your own physiology. And if bumping it up to 30-35 miles week doesn't carry over to the 800m, at least some gains, then jumping to 60/week wont be the answer anyway. If you do go for a bigger volume base, you still want to be able to bike over to the track, put on the spikes and fairly casually hit a 51 for 400m. If you lose more speed than that with the added over distance, you are robbing Peter to pay Paul.